Water and air are indispensable to survival of any form of life on the earth, including human kind, and their availability and quality exert a great influence on the sustenance of the earth as well as humans. With the advance of scientific and industrial development, the applicability of water resources is becoming more and more abundant and water resources are becoming increasingly important. Under the circumstances, acquisition of high-quality water resources is of great concern for the sustenance of lives in many areas around the world. Although the world face scarcity and contamination of water resources and there is a continuously growing need for water supply, available water resources are not readily accessible. According to United Nation's surveys, there are over 120 million people, about one fifth of the global population, have great difficulty in accessing to safe drinking water, and the number of people facing scarcity of sewage treatment facilities has doubled, that is, about 240 million people are drinking poor quality drinking water. Today there are over 300 million people around the world die everyday because of unsanitary drinking water, which results from poorly managed water resources.
Reportedly, over 70% of available water in the world is seawater. However, since sea water contains a great deal of impurities such as salinity, various kinds of salt solids, or the like, it cannot be directly used as potable water for industrial, agricultural or home use. Accordingly, in order to allow people to avail themselves of sea water or saline water in a wide variety of areas in their daily lives, desalination is essentially performed to remove various salts from the sea water or saline water. A TFC reverse osmosis membrane is essentially used in desalination.
A general thin film composite (TFC) reverse osmosis membrane comprises a porous polymer support that offers a mechanical strength and a thin active layer formed on the porous polymer support. In particular, a polyamide active layer is formed at an interface between polyfunctional amine aqueous solution and polyfunctional acyl halide organic solution. An exemplary polyamide TFC reverse osmosis membrane is described in U.S. Pat. No. 4,277,344, which was issued to Cadotte in 1981. According to this patent, an aromatic polyamide active layer is formed through an interfacial polymerization reaction occurring between polyfunctional aromatic amine monomer having at least two primary amine groups polyfunctional acyl halide monomer having at least three acyl halide groups. In the described technology, interfacial polymerization is carried out such that a polysulfone support is immersed into an 1,3-phenylenediamine (MPD) aqueous solution, excess 1,3-phenylenediamine (MPD) aqueous solution on a surface of the support is removed and the resultant product is coated with a trimesoyl chloride (TMC) dissolved in “FREON” TF (trichlorotrifluoroethane) solvent. Here, a contact time for the interfacial polymerization is 10 seconds. After completing the interfacial polymerization, the resulting TFC reverse osmosis membrane is dried at room temperature. The TFC reverse osmosis membrane prepared by the Cadotte method exhibited relatively good flux and good salt rejection performance. Various approaches have been taken thereafter to further improve the flux and salt rejection performance of polyamide TFC reverse osmosis membranes.
For example, in U.S. Pat. No. 4,872,984, which was issued to Tomaschke in 1989, there is disclosed a polyamide TFC reverse osmosis membrane formed by interfacial polymerization performed such that an aqueous solution containing polyfunctional aromatic amine monomer having at least two reactive amine groups and a amine salt compound prepared by monomeric tertiary amine and strong acid is contacted with an organic solution containing aromatic polyfunctional acyl halide compound on a porous support. Here, the amine salt compound contained in the aqueous solution is a monomeric amine, which consists of a tertiary amine salt formed by a tertiary amine and a strong acid or a quaternary amine salt. Among the amines used herein, examples of the useful monomeric tertiary amines include a trialkylamine, such as trimethylamine, triethylamine, tripropylamine; an N-alkylcycloaliphatic amine, such as 1-methylpiperidine; an N,N-dialkylamine, such as N,N-dimethylethylamine and N,N-diethylmethylamine; an N,N-dialkyl ethanolamine, such as N,N-dimethylethanolamine; and so on. Examples of the quaternary amine salts include a tetraalkylammonium hydroxide, such as, tetramethylammonium hydroxide, tetraethylammonium hydroxide, and tetrapropylammonium hydroxide; a benzyltrialkylammonium hydroxide, such as benzyltrimethylammonium hydroxide, benzyltriethylammonium hydroxide, and benzyltripropylammonium hydroxide; and mixtures thereof.
In U.S. Pat. No. 5,576,057, issued to Hirose in 1996, there is described a TFC reverse osmosis membrane having improved flux by adding 10˜50 wt % alcohol to an amine aqueous solution. Here, preferred examples of the alcohol used include ethanol, propanol, botanol, butyl alcohol, 1-pentanol, 2-pentanol, isobutyl alcohol, isopropyl alcohol, 2-ethylbotanol, 2-ethylhexanol, octanol, cyclohexanol, tetrahydrofurfuryl alcohol, neopentyl glycol, t-botanol, benzyl alcohol, 4-methyl-2-pentanol, 3-methyl-2-butanol, pentyl alcohol, allyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propanediol, butanediol, pentanediol, hexanediol, glycerol, or a combination thereof. However, when alcohol is added to a first amine solution to prepare the TFC reverse osmosis membrane, a solubility difference between the solubility parameter of the first amine solution and the solubility parameter of a second organic solution should be from 7 to 15 (cal/cm3)1/2. If the solubility difference between the solubility parameter of the first amine solution and the solubility parameter of the second organic solution is greater than 15 (cal/cm)1/2, the polyamide active layer is formed well at the interface between the two solutions by interfacial polymerization but the water permeability is decreased.
The thus-formed polyamide TFC reverse osmosis membrane exhibited 29˜42 [L/m2 hr] in water permeability and 99.4˜99.5% in salt rejection. That is to say, the polyamide TFC reverse osmosis membrane had improved water permeability compared to a conventional polyamide reverse osmosis membrane prepared without alcohol added, the conventional polyamide reverse osmosis membrane having 25 [L/m2hr] of water permeability and 99.6% of salt rejection. In this case, when a small amount of alcohol is used, the alcohol adding effect is negligible. On the other hand, when excess alcohol is used, an interfacial polymerization reaction may not be properly carried out due to a similar solubility parameter between the solubility parameter of the amine aqueous solution and the solubility parameter of an organic solution of acyl halide. Thus, the thus-formed polyamide TFC reverse osmosis membrane has an undesirably reduced salt rejection.
In U.S. Pat. No. 4,950,404, issued to Chau et al., there is proposed a method for forming a polyamide TFC reverse osmosis membrane by interfacial polymerization method, in which to a polar aprotic solvent is added to an amine aqueous solution and is made to be in contact with an organic solution containing polyfunctional acyl halide on a surface of a porous support. Here, preferred examples of the polar aprotic solvent used include N-methylpyrrolidone, 2-pyrrolidone, N,N-dimethylformamide, dioxane, pyridine, lutidine, picoline, tetrahydrofuran, sulforan, sulforene, hexamethylphosphoamide, triethylphosphite, N,N-dimethyacetqamide, N,N-dimethypropionamide, and the like.
Chau, et al. proposed a polyamide TFC reverse osmosis membrane, as disclosed in U.S. Pat. No. 4,983,291. The polyamide TFC reverse osmosis membrane was prepared through post-treatment performed by contacting an polyamide active layer prepared by interfacial polymerization on a porous support with a solution containing an acid such as ascorbic acid, hydrochloric acid, citric acid, sulfamic acid, tartaric acid, ethylenediaminetetraacetic acid, p-toluenesulfonic acid, L-lysine hydrochloride, or glycine, followed by drying the resultant product at a given temperature (ranging from room temperature to 170° C.) for a period of time (1 min˜120 min). However, as a composition ratio of the aprotic solvent increases to attain high water permeability, the prepared polyamide TFC reverse osmosis membrane exhibited a reduced degree of salt rejection of the reverse osmosis membrane. In a case where the prepared TFC reverse osmosis membrane was allowed to contact with an acid-containing solution and dried at about 100° C., if the acid was added in a small amount, an effect of enhancing the performance of the membrane was negligible. Conversely, if an excessive amount of acid was added, the salt rejection of the membrane was reduced while enhancing the water permeability of the membrane. In a case where the polyamide TFC reverse osmosis membrane was contacted with an acid-containing solution and dried at relatively high temperature of about 170° C., its water permeability was reduced.
Ja-Young Koo, et al. proposed a polyamide TFC reverse osmosis membrane, as disclosed In U.S. Pat. No. 6,245,234, issued to in 2001. In the disclosed patent, in order to improve the water permeability of the polyamide TFC reverse osmosis membrane, a polyfunctional tertiary amine and a strong acid were added to an amine aqueous solution, forming a polyfunctional tertiary amine salt, the polyfunctional tertiary amine comprising a hydrocarbon alkane backbone having at least two tertiary amine groups. In addition, in order to improve water permeability of the polyamide TFC reverse osmosis membrane, a polar solvent is further added. The polyfunctional tertiary amine added to the aqueous amine solution is preferably selected from the group consisting of N,N,N′,N′-tetramethyl-1,6hexanediamine, N,N,N′,N′-tetramethyl-1,4-buthanediamine, N,N,N′,N′-tetramethyl-2-butene-1,4-diamine, N,N,N′,N′-tetramethyl-1,3-buthanediamine, N,N,N′,N′-tetramethyl-1,3-propanediamine, N,N,N′,N′-tetramethyl-1,8-octanediamine, N,N,N′,N′-tetramethyl-1,7-heptanediamine, N,N,N′,N′-tetramethyl-1,5-pentanediamine, N,N,N′,N′-tetraethyl-1,4-buthanediamine, N,N,N′,N′-tetraethyl-1,3-buthanediamine, N,N,N′,N′-tetraethyl-1,3-propanediamine, 1,4-dimethylpiperazine, N,N,N′,N′-tetraethylethylenediamine. Here, the polyfunctional tertiary amine and the strong acid are reacted together in a molar ratio that is greater than 1:0, respectively, and is less than 1:n, respectively. In addition, preferred examples of the polar solvent include 2-methoxyethanol, 2-ethoxyethanol, 2-propoxyethanol, 2-butoxyethanol, t-butylmethyl ether, 1,3-heptanediol, 2-ethyl-1,3-hexanediol, dimethyl sulfoxide, tetramethyl sulfoxide, butyl sulfoxide, and methylphenyl sulfoxide.
In U.S. Pat. No. 6,368,507, issued to Koo, et al. in 2002, there is proposed a polyamide TFC reverse osmosis membrane prepared by a reaction of an amine aqueous solution and an organic solution, the amine aqueous solution comprising a salt compound containing a polyfunctional amine, a polar solvent, a tertiary amine salt, and a tertiary amine, and the organic solution comprising a polyfunctional acyl halide, a polyfunctional sulfonyl halide or a polyfunctional isocyanate, wherein a molar ratio of the polyfunctional amine to a strong acid is greater than or equal to 1:1, respectively, and is less than 1:n, respectively, n being the number of amine groups in the polyfunctional tertiary amine.
In U.S. Pat. No. 5,755,964, issued to Mickols et al. in 1998, there is disclosed a polyamide TFC reverse osmosis membrane. In the disclosed patent, to improve the water permeability of the reverse osmosis membrane, a polyamide active layer is treated with ammonia or a particular alkyl amine. That is, the disclosed patent proposed a method of increasing the flux of a TFC reverse osmosis membrane or a polyamide nanofiltration (NF) membrane. Usable examples of the amine proposed by the disclosed invention include ammonia substituted with one to three alkyl groups of one to two carbons which alkyl groups may be further substituted with one or more substitutents selected from hydroxy, phenyl, or amino; butylamine; cyclohexylamine; 1,6-hexanediamine and mixtures thereof. In addition, preferred substituted ammonia substances include dimethylamine; trimethylamine; ethylamine; triethanolamine; N,N-dimethyl ethanolamine; ethylenediamine; and benzylamine. The patent proposed by Mickols et al. is directed to a method of preparing a nanofiltration membrane or a reverse osmosis membrane including preparing a polyamide TFC reverse osmosis membrane by an interfacial polymerization reaction such that a porous support surface is contacted with an amine aqueous solution containing polyfunctional aromatic amine monomer and an organic solution containing polyfunctional acyl halide monomer, contacting a polyamide active layer with the amine aqueous solution selected from the group consisting of an amine from the group consisting of ammonia; ammonia substituted with one to three alkyl groups of one to two carbons which alkyl groups may be further substituted with one or more substitutents selected from hydroxy, phenyl, or amino; butylamine; cyclohexylamine; 1,6-hexanediamine and mixtures thereof. Here, the concentration of the amine aqueous solution is in the range of about 5 to about 50 wt %. In addition, the proposed method consists essentially of contacting the discriminating layer with the amine having a pH level in the range of about 7 to about 12 at a temperature of 0 to 130° C. for 15 seconds to 5 days. Further, according to the method proposed by Mickols et al., the polyamide active layer is further dried at a temperature of from about 60 to about 80° C. after contact with the amine. The thus prepared composite membrane may be a reverse osmosis membrane or a nanofiltration membrane.